Conventional inkjet printers employ a print head assembly having an array of individual nozzles for deposition of ink onto a substrate media or material e.g., plain white paper or a mailpiece envelope. The print head assembly is adapted to accommodate one of two types of print/feed mechanisms, namely, shuttle and in-line printing. Shuttle motion printers employ a moveable print head assembly capable of shuttling back and forth in a direction orthogonal to the direction of paper feed. Consequently, shuttle print head assemblies are capable of fully covering the printable area of a page in bands of coverage. In-line printing, in contrast, employs a stationary print head assembly having a fixed width. Generally, in-line print head assemblies comprise a plurality of adjacent print heads to cover a predetermined print area or region as paper is fed beneath the print heads.
In FIGS. 1a and 1b a conventional prior art shuttle print head assembly 100 comprises a carriage assembly 110 for containing one or more individual print heads 112. The carriage assembly 110 is typically connected to a guide rail 114 to fix the print head 112 in two axes and a positioning mechanism (not shown) to control movement in an axis perpendicular to the feed path (in the direction of arrow FP) of the media 116. As mentioned in a preceding paragraph, the shuttle print head assembly 100 is capable of covering the entire face surface of the media 116, i.e., bi-directionally along arrow SP (see FIG. 1a), by incrementally moving media 116 through the print station as the print head 112 passes back and forth in bands of coverage.
To ensure that the print head 112 prints on a substantially planar surface, it is common to employ a movable shield 120 proximal to the print head 112 to press the media 116 against an underlying support structure 122, i.e., typically a resilient elastomer roller or feed cylinder. Furthermore, the shield 120 may be adjusted vertically along bi-directional arrows AV to accommodate thickness variations in print media 116 such as mailpiece envelopes. Due to the bi-directional motion SP of the print head assembly 100 and the direction FP of the feed path, the shield 120 must necessarily be disposed behind the print head 112 and parallel its shuttle motion SP. While the shield 120 is as close as practicable to the print head 112, it will be appreciated that the ability to control the surface contour of the print media diminishes as the distance from the shield increases. Consequently, as best seen in FIG. 1b, the shield 120 exerts less influence over a region D which distally located as compared to region A which is proximal to the shield 120.
In FIG. 1c, an in-line print head assembly 200 is depicted wherein the carriage assembly 210, including the print head 212, is essentially fixed relative to a supporting rail 214. The underlying print media 216 is conveyed along feed path a FP by linear belts 218 which pass beneath the print head 212. Such in-line print heads 212 typically employ a fixed shield 220 disposed along the leading edge LE of the print head 212, i.e., laterally of the print head 212, and in-advance of the print media 216. Further, to minimize complexity, the fixed shield 220 is typically rigidly mounted to the carriage assembly 210, near the print head 212, and spring biased against the print media 216.
The shield 220 may be configured and mounted in the manner described above principally due to the unidirectional motion of the underlying print media 216. That is, inasmuch as the ink requires a fixed amount of time, albeit small, to dry, the shield 220 may be located along the leading edge LE or upstream of the print head 212, to avoid contact with the drying ink. Additionally, the shield 220 may optimally positioned along side the print head 212 to further improve the efficacy of printing, i.e., controlling the surface contour in region A with greater precision.
While the mounting arrangement of the shield 220 is optimally suited for in-line printing assemblies 200, i.e., wherein the print media 216 moves in a single direction relative to the print head 212, it will be appreciated that a similar mounting arrangement is not possible for shuttle motion printers. That is, should a pair of shields be disposed on each side of a shuttling print head, one of the shields, i.e., the shield disposed downstream of ink deposition will interfere with the drying ink.
A need, therefore, exists for a shield assembly which optimally controls the surface contour of underlying print media while enabling both in-line and shuttle ink-jet printing.